L5-Microbial PART 2 Metabolism: Aerobic respiration

• Oxidative Phosphorylation


  The metabolic pathway in which cells use enzymes to oxidise nutrients, thereby releasing chemical energy to produce adenosine triphosphate (ATP)

  
• Oxidative Phosphorylation


  • Made up of two closely connected components: the electron transport chain and chemiosmosis
  • The electron transport chain in the cell is the site of oxidative phosphorylation
  • The NADH and succinate generated in the citric acid cycle are oxidized, releasing the energy of O2 to power the ATP synthase

  
• Electron carriers


  Small organic molecules that play key roles in cellular respiration by picking up electrons from one molecule and dropping them off with another

  
• NAD (nicotinamide adenine dinucleotide)


  An electron carrier that when it picks up electrons, also gains one or more hydrogen atoms, switching to a slightly different form (NAD+ + 2e- + 2H+ → NADH + H+)

  
• FAD (flavin adenine dinucleotide)


  An electron carrier that when it picks up electrons, also gains one or more hydrogen atoms, switching to a slightly different form (FAD + 2e- + 2H+ → FADH2)

  
• Redox reactions


  Reactions in which NAD+ and FAD gain or lose electrons
• Cytochromes


  Hemoproteins that undergo electron transfer with the help of an enzyme and play a vital role as electron carriers in mitochondria

  
• Cytochromes


  • Classified into a, b, c based on light-absorption spectra
  • Over 30 identified variants
  • Cytochrome c is the most stable and abundant member and is extensively studied and crucial in cellular respiration

  
• Ferredoxin (Fd)


  Small proteins with one or two iron–sulphur clusters that are generally involved in oxidoreduction reactions and act as electron carriers with low redox potential in electron transport chains

  
• Ferredoxin (Fd)


  • Prosthetic groups with iron and sulphur atoms in three centre types: 2Fe–2S, 4Fe–4S, and 3Fe–4S
  • Integral to fundamental metabolic processes like photosynthesis, nitrogen fixation, hydrogen, nitrogen, and sulphur assimilation

  
• Quinones


  Small hydrophobic redox molecules without a protein component that can move within the membrane and transfer 2 electrons to the next carrier in the chain

  
• Quinones


  • Types include ubiquinone (coenzyme Q) and menaquinone which are common and widely distributed in bacteria and archaea species

  
• Electron Transport Chain (ETC)


  A series of compounds utilizing electrons from electron carriers to establish a chemical gradient that can fuel oxidative phosphorylation

  
• Electron Transport Chain (ETC)


  • Consists of enzymes, including protein complexes, proteins, peptides, and more
  • Highly efficient method for generating large amounts of ATP by transferring electrons from electron donor to electron acceptor, leading to ATP production

  
• Complex 1 (NADH:quinone oxidoreductase or NADH dehydrogenase)


  Oxidizes NADH to NAD+, resulting in 2 electrons (e–) that are transferred to FMN and Fe-S, then to ubiquinone (Q) which accepts 2 e– and 2 H+ from the cytoplasm to become ubiquinol (QH2), and transports 4 H+ across the membrane

  
• Complex 2 (Succinate dehydrogenase)

  Directly receives FADH2 not passed through Complex 1, and oxidation of succinate to fumarate in the citric acid cycle reduces FAD to FADH2, with 2 e– from FADH2 transferred through Complex II to Q
• Complex 3 (Q-cycle)


  2 e– from QH2 enter at the Qo site, 1 e– is donated to cytochrome c, then 2 H+ are released, contributing to proton motive force, while the 2nd e– is sent to Cytochrome b (subunits L and H), then to Q at the Qi site, forming a semi-quinone radical (Qe–)

  
• Complex 4 (Cytochrome oxidase)


  Functions as the terminal oxidase, containing cytochromes a and a3, receives 2 e– from cytochrome c, reduces O2 to 2H2O, and pumps out 2 H+ from the cytoplasm

  
• ATP Synthase


  The mitochondrial enzyme that converts ADP and phosphate into ATP, with the F1 (catalytic) complex catalyzing the reaction and the F0 (rotor) complex carrying out proton translocation across the membrane

  
• ATP Synthase


  • For every full rotation of the c ring within the F0 subunit, three ATP are formed by the F1 subunit
  • Proton motive force (PMF) is the driving force for ATP synthesis by ATP synthase, as protons flow through a channel in the enzyme causing mechanical movement that provides the energy to add phosphate to ADP

  
• ATP synthase can run in reverse, with ATP hydrolysis driving the ejection of protons out of the cytoplasm to generate PMF

  
• Some bacterial and archaeal ATPases are linked to a sodium (Na+) rather than a proton (H+) gradient
• Oxidative phosphorylation is a highly efficient method for generating large amounts of ATP compared to other stages in cellular respiration

  
• Proton motive force (PMF) is crucial in cellular respiration as the driving force for ATP synthesis by ATP synthase